I just saw this thread. Really, much depends on what kind of portraiture you intend to do. As Keith said, the holidays are great for the portrait business, but can leave January and February a little flat. If, on the other hand, you are planning to do maternity / newborn photography, your business will be more regular.
If you're going to launch your business before spring, I would do it immediately. Keeping in mind lead times in the busy season, you're already a little behind for holiday business.